Polymer electrolyte fuel cell

ABSTRACT

A polymer-electrolyte fuel cell having: an electrolyte membrane-electrode assembly having a polymer-electrolyte membrane and a pair of gas-diffusion electrodes sandwiching the polymer-electrolyte membrane; a first electroconductive separator having a gas channel for supplying an oxidant gas to one of the gas-diffusion electrodes of said pair; and a second electroconductive separator having a gas channel for supplying a fuel gas to the other of the gas-diffusion electrodes of the pair. The polymer-electrolyte fuel cell is characterized in that: at least one of the first and second electroconductive separators has a metal substrate and an electroconductive resin layer on the substrate and contacting the electrolyte membrane-electrode assembly; and the electroconductive resin layer has a resin having water-repellant or basic radicals, and an electroconductive particulate substance which has a carbon powder of a specific surface area of less than 100 m 2 /g.

TECHNICAL FIELD

The present invention relates to polymer-electrolyte fuel cells usefulas consumer co-generation systems and mobile power-generatingappliances.

BACKGROUND ART

In the gas-diffusion electrodes of fuel cells, a fuel gas such ashydrogen and an oxidant gas such as air react electrochemically, so asto supply electricity and heat simultaneously. Owing to the variety ofelectrolytes with which they are equipped, there are several types offuel cells.

Polymer-electrolyte fuel cells are furnished with electrolytemembrane-electrode assemblies (MEAs), made up of a polymer-electrolytemembrane and a pair of gas-diffusion electrodes sandwiching themembrane. The polymer-electrolyte membrane for example has a —CF₂—skeleton and comprises a perfluorocarbon sulfonic acid having sulfonicacids on the terminal ends of its side chains.

The gas-diffusion electrodes comprise a catalyst layer contiguous withthe polymer-electrolyte membrane and, arranged on the outer face of thecatalyst layer, an electrode substrate having gas-permeable andelectroconductive properties. The catalyst layer comprises a carbonpowder carrying a platinum-system metal.

An electroconductive separator for affixing an MEA, and at the same timeelectrically interconnecting in series neighboring MEAs, is arranged onthe outer face of the MEA. The electroconductive separator has agas-supplying channel for the fuel gas or the oxidant gas to thegas-diffusion electrodes, and for conveying a surplus gas and watercreated by the reaction of hydrogen and oxygen.

Seals such as gaskets are arranged encompassing the gas channels on theelectroconductive separator and the gas-diffusion electrodes, and theyprevent intermixing and outward leakage of the gases.

To heighten output voltage in procuring power-generating devices, aplurality of individual single cells, composed of an MEA and a pair ofelectroconductive separators having gas channels, are laminated. A fuelgas and an oxidant gas are supplied from the exterior through a manifoldto each gas channel. Electric current generated through the electrodereactions is then collected at the electrode substrates and taken out tothe exterior through the electroconductive separator.

Electroconductive separators are often made from a carbon materialhaving gas-tight and anticorrosive properties. Likewise,electroconductive separators utilizing a metal substrate such asstainless steel have been investigated from the viewpoints ofmanufacturing cost, as well as ease of working and thinning theelectroconductive separator.

Due to long-term exposure to high-humidity gases, electroconductiveseparators utilizing metal substrates require strong corrosionresistance. Furthermore, in order to heighten the electric cells'power-generating efficiency, suppressing contact resistance between theelectroconductive separator and the MEA also becomes important. Therein,if for example a stainless-steel sheet is utilized as a metal substrate,by forming a passive state layer consisting of chromium oxide on theobverse face of the stainless-steel sheet, its corrosion resistance isheightened.

Nevertheless, because forming the comparatively thick and stable passivestate layer on the obverse face of the metal substrate makes the passivestate layer an electrical resistor, the contact resistance increases.High-output cells consequently cannot be procured. Conversely, if thepassive state layer is unstable, the metal substrate will corrode, andthe MEA will undergo damage due to the metal ions leached.

A method of establishing on the obverse face of the metal substrate alayer obtained by means of chemical-plating or vapor-depositing ananticorrosive metal such as gold has been investigated. Lowering costs,however, is difficult.

A method of coating onto the obverse face of the metal substrate a resincomposition in which a carbon powder is dispersed into cellulose,poly(vinyl chloride), epoxy resin, etc. has also been investigated.Nevertheless, there are problems with the durability.

A method of forming a layer on the obverse face of the metal substratein order to heighten its corrosion resistance, and meanwhile arranging,in the area where the separator and the MEA contact, anelectroconductive particulate substance of high hardness in order toform an electroconductive path by penetrating the aforementioned layer,has also been investigated. This method is comparatively low-cost.Nevertheless, when the cells are run long-term, in the end metal ionsleach out and the MEA undergoes damage.

DISCLOSURE OF INVENTION

The present invention relates to a polymer electrolyte fuel cell made upof: an electrolyte membrane-electrode assembly comprising apolymer-electrolyte membrane, and a pair of gas-diffusion electrodessandwiching the polymer-electrolyte membrane; a first electroconductiveseparator having a gas channel for supplying an oxidant gas to one ofthe gas-diffusion electrodes of the pair; and a second electroconductiveseparator having a gas channel for supplying a fuel gas to the other ofthe gas-diffusion electrodes of the pair; wherein thepolymer-electrolyte fuel cell is characterized in that: at least one ofthe first electroconductive separator and the second electroconductiveseparator comprises a metal substrate and an electroconductive resinlayer provided on the metal substrate and contacting the electrolytemembrane-electrode assembly; and the electroconductive resin layercomprises a resin having water-repellant or basic radicals, and anelectroconductive particulate substance.

Here, on the one surface of the first electroconductive separator or thesecond electroconductive separator there may be a gas channel forsupplying an oxidant gas to one of the gas-diffusion electrodes of oneMEA, and on the other surface there may be a gas channel for supplying afuel gas to one of the gas-diffusion electrodes of another MEA.

The electroconductive particulate substance may preferably comprise acarbon powder having a specific surface area of less than 100 m²/g.

Further, the electroconductive particulate substance may preferablycomprise vitreous carbon.

A polymer-electrolyte fuel cell in accordance with the present inventionmay preferably have, between the metal substrate and theelectroconductive resin layer, a layer including at least one selectedfrom the group consisting of: metallic Zn, metallic Sn, metallic Al,Cr-containing compounds, Mo-containing compounds and W-containingcompounds.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of contacting portions of anelectroconductive separator and a gas-diffusion electrode relating tothe present invention; and

FIG. 2 is a plot indicating results of a durability test on a singlecell manufactured in an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The oxidant gas and fuel gas supplied to the polymer-electrolyte fuelcell must be humidified in order to sustain the conductivity of thepolymer-electrolyte membrane. Likewise, water is produced at the cathodeby galvanic reaction. Accordingly, water vapor is thought to condense,while the cell is in operation, in the area where the electroconductiveseparators and the gas-diffusion electrodes contact. Carbon dioxide gasand ionic impurities then dissolve in the condensed water vapor.Corrosion of the metal substrate in the electroconductive separatordevelops as a result.

The electroconductive separator of the present invention is made up of ametal substrate, and electroconductive resin layer provided on thesurface of the metal substrate. Then the gas-diffusion electrodes arecontacted on the electroconductive resin layer. The electroconductiveresin layer is made from a resin having water-repellant or basicradicals that are dispersed with an electroconductive particulatesubstance. Here, the resin may include both water-repellant radicals andbasic radicals.

When utilized, the resin having water-repellant radicals in theelectroconductive resin layer owing to its water repellency keepsionically conductive water from permeating the electroconductive resinlayer and reaching the surface of the metal substrate. Accordingly, evenif pinholes might be present in the electroconductive resin layer,corrosion due to water passing through them and coming into contact withthe metal substrate is restrained.

Fluorine-atom-containing radicals such as —F, —CF₃, —CF₂— andpentafluorophenyl group may be cited as water-repellent radicals.

Meanwhile, the corrosion resistance of alloys such as stainless steel isheightened by a passive state oxide layer formed on the surface. Thestability of the passive state layer varies according to the ambient pH.The passive state layer will corrode in acidic atmospheres, and theamount of ions that leach from stainless steel will be large. Leachedmetal ions enter polymer-electrolyte membranes in place of hydrogenions, deteriorating the hydrogen-ionic conductivity of the membranes.Likewise, metal ions are thought to lower the water content ofpolymer-electrolyte membranes, and to impair the catalytic activity ofplatinum.

In that respect, utilizing in the electroconductive resin layer a resinhaving basic radicals heightens the surficial pH of the separator, andstabilizes the passive state layer. Consequently, leaching of metal ionswhile the cell is in operation is restrained for an extended period.

Nitrogen-atom-containing radicals such as amino groups, amide groups,imide groups may be cited as basic radicals. Wherein the resin includesthese radicals, the corrosion resistance of the electroconductiveseparator is thought to improve because the oxide layer on the adjoiningmetal surface is stabilized by an electron-pair of the nitrogen atom.

Further, leaching of metal ions from the metal substrate is restrainedby arranging on the surface of the metal substrate a layer containing ametal, such as Zn, Sn or Al, whose ionization tendency is greater thanthe metal in the metal substrate. Likewise, leaching of metal ions fromthe metal substrate is also restrained by forming onto themetal-substrate surface a layer comprising an oxide or hydroxide of Cr,Mo, W or the like.

In order to gain high electroconductivity, a sufficient quantity of theelectroconductive particulate substance must be incorporated in theelectroconductive resin layer. Nevertheless, because metal ions createdin the surface of the metal substrate leach to the exterior of theelectroconductive separator due to dispersing on the interface betweenthe resin and the electroconductive particles, the specific surface areaof the electroconductive particulate substance desirably should besmall.

Powders of metal oxides, metal nitrides, metal carbides or the like thatare to a certain extent anticorrosive are effective as theelectroconductive particulate substance. For example, powders of carbon,titanium, ruthenium oxide, titanium nitride or titanium carbide areeffective. Moreover, among these a carbon powder in particular iseffective.

The carbon powder preferably consists of layered graphite, flakegraphite or vitreous carbon. Further, among these a vitreous-carbonpowder is preferable. This is because metal ions are thought to dispersethrough the spacing of layers in the layered graphite and leach to theexterior. Likewise, mixing together and utilizing a highlyelectroconductive layered graphite powder and an amorphous carbon powdersuch as a vitreous carbon powder is preferable in terms of balancebetween electroconductivity and control of metallic ion elution.

The amount of electroconductive particulate substance contained in theelectroconductive resin layer is, generally, 100–900 parts by weight per100 parts by weight resin.

Because the passive state layer on the surface of the metal substratewill cause an increase in the contact resistance, in situations wherecorrosion is not a problem, the passive state layer preferably iseliminated prior to forming the electroconductive resin layer. Here, inpoint of corrosion resistance the metal substrate preferably comprisesstainless steel or carbon steel. A metal substrate comprising a metal oflow corrosion resistance, such as aluminum, may be utilized for thepresent invention, however.

Examples of electrode substrates that may be given are: carbon papers;carbon clothes obtained by weaving carbon fibers; and carbon feltsobtained by molding a mixture of carbon fibers and carbon powders towhich a fibrous binder is added.

As a polymer-electrolyte membrane, those that have conventionally beenutilized in the MEAs of polymer-electrolyte fuel cells may be used.

EMBODIED EXAMPLE 1

A catalyst powder in which 75 parts by weight acetylene black (180 m²/gspecific surface area) carries 25 parts by weight platinum particles(approx. 30 Å mean particle diameter) was prepared. A paste was obtainedby mixing a dispersion liquid consisting of this catalyst powder andisopropanol, and a dispersion liquid consisting of the perfluorocarbonsulfonic acid powder indicated by chem. formula (1) and ethanol. Theperfluorocarbon sulfonic acid inclusion quantity is 30 parts by weightper 100 parts by weight catalyst powder.Formula (1):

Next, a carbon paper 300 μm in thickness, which would become thesubstrate for the gas-diffusion electrodes, was prepared. Then, on onesurf ace thereof a layer 40 μm in thickness of a carbon powder 180 m²/gspecific surface area), repellency-treated with polytetrafluoroethylene(PTFE) in an aqueous dispersion (PTFE inclusion quantity was 200 partsby weight per 100 parts by weight carbon powder), was provided.Subsequently, by applying the aforementioned paste onto this layer, acatalyst layer approximately 30 μm in thickness was built to yield agas-diffusion electrode.

A polymer-electrolyte membrane was sandwiched in a pair of thegas-diffusion electrodes, putting the catalyst layers inwards, andhot-pressed 30 sec. at 110° C., to obtain an MEA. “Nafion 112” (Du PontCorp., mfr.; thickness: 50 μm) was utilized as the polymer-electrolytemembrane.

Next, 2 stainless-steel sheets (SUS 316) 500 μm in thickness wereprepared. Likewise, a resin composition for building theelectroconductive resin layer was prepared. The resin composition wasobtained by knead-blending, in a planetary ball-mill, a mixture of afluoroelastomer (copolymer powder composed of CF₂═CF—CF═CF₂, CH₃—CF═CF₂,CF₂═CF₂ or the like), flake graphite, and methylethyl ketone (10:50:40weight proportion). The fluoroelastomer has water-repellent properties.This resin composition was applied and dried onto one side of thestainless-steel sheets, forming on each an electroconductive resin layer15 μm in thickness. Subsequently, a hydrogen gas channel was formed onone of the stainless-steel sheets—in its surface having theelectroconductive resin layer—and an air channel was formed on the otherstainless-steel sheet—in its surface having the electroconductive resinlayer—which yielded a pair of electroconductive separators. The gaschannels were built by pressing the stainless-steel sheet having theelectroconductive resin layer to form gas-conducting groove or rib, andassembling this with an insulating sheet having elasticity. Theinsulating sheet was formed by a punching process, and a channel wasformed that, cooperating with the aforementioned groove in or rib on thestainless-steel sheet, leads gases from its supply-side to its dischargeside, while functioning as a gasket that prevents gases from leakingfrom the aforementioned channel to the exterior (see W000/01025).

The MEA, peripherally on which a silicone rubber gasket was arranged,was sandwiched with the obtained pair of electroconductive separators toobtain a single cell. At this time, the electroconductive resin layerson the electroconductive separators and the outer surfaces of the MEAgas-diffusion electrodes were brought into contact.

In this cell, the electricity generated by the MEA may be taken out tothe exterior by way of the surfaces of the gas-diffusion electrodes thatcontact with the electroconductive separator, and the electroconductiveseparator. In practice, a laminated cell is formed in which a pluralityof cells of this sort and cooling cells for flowing a cooling mediumsuch as cooling water are laminated, but in the present embodiment, theexample of a single cell is explained.

FIG. 1 is a sectional schematic diagram of contacting portions of anobtained electroconductive separator and gas-diffusion electrode. InFIG. 1, 1 is the stainless-steel sheet (SUS 316), and 2 indicates theelectroconductive resin layer built on the surface thereof.Electroconductive particles (flake graphite particles) 3 for securingelectroconductive properties are dispersed within the electroconductiveresin layer 2. The electroconductive particles 3 are in mutual contact,or are adjacent enough to have electroconductivity. The carbon paperpart of the gas-diffusion electrode is 4; and 5 is a carbon fiber thatconstitutes the carbon paper.

Water 7 that is condensed water vapor is present on the interface 6between the carbon paper and the electroconductive resin layer, butcontact between the water 7 and the stainless-steel sheet 1 is blockedby the electroconductive resin layer 2. Moreover, because theelectroconductive resin layer 2 is water repellent, though a pinhole 8is present in the electroconductive resin layer 2, the water 7 cannotpass through the pinhole 8.

Durability testing was next carried out on the obtained single cell.Here, the electrode surface area is 25 cm² (5-cm square), and 0.3 mg/cm²Pt is contained respectively in the catalyst layers on the cathode andthe anode. Hydrogen gas was supplied to the anode, and air to thecathode. Likewise, the cell temperature was set to 75° C., the fuelconsumption rate to 70%, and the air consumption rate to 30%.Furthermore, the gases were humidified so that the dew point of thehydrogen gas would be 75° C., and of the air, 60° C. The relationshipbetween cell voltage and running time when the single cell was operatedcontinuously at a current density of 0.5 A/cm² is shown by a in FIG. 2.

EMBODIED EXAMPLE 2

A resin composition was obtained by knead-blending while adding to flakegraphite an aqueous dispersion of a copolymer of tetrafluoroethylene andhexafluoropropylene, and partially vaporizing the water. Copolymers oftetrafluoroethylene and hexafluoropropylene are water repellent. Theweight proportion of the flake graphite to the resin component in thisresin composition was 50:10. An electroconductive separator wasfabricated that, apart from utilizing this resin composition, was thesame as in Embodied Example 1, and the same evaluation was made. Theresults are shown by b in FIG. 2.

COMPARATIVE EXAMPLE 1

A resin composition was obtained by knead-blending in a planetaryball-mill a mixture composed of poly(vinyl chloride), flake graphite,and methyl ethyl ketone (10:50:40 weight proportion). Anelectroconductive separator was fabricated that, apart from utilizingthis resin composition, was the same as in Embodied Example 1, and thesame evaluation was made. The results are shown by c in FIG. 2.

COMPARATIVE EXAMPLE 2

A resin composition was obtained by knead-blending in a planetaryball-mill a mixture composed of polymethylacrylate, flake graphite, andwater (5:45:50 weight proportion). An electroconductive separator wasfabricated that, apart from utilizing this resin composition, was thesame as in Embodied Example 1, and the same evaluation was made. Theresults are shown by d in FIG. 2.

As is clear from FIG. 2, voltage deteriorated in a short time intervalfollowing the start of operation with the cells in which the poly(vinylchloride) and acrylic resin were utilized in the electroconductive resinlayer. On the other hand, with the cells in which the fluoroelastomerand fluoropolymer were utilized, no significant drop in voltage wasrecognized even after the elapse of a long time interval.

Therein, in order to ascertain the water repellency of theelectroconductive resin layer, the water contact angle was measured insmooth, flat locations on each electroconductive resin layer. The resultwas that the electroconductive resin layer for which the contact anglewas largest was that in which the copolymer of tetrafluoroethylene andhexafluoropropylene was utilized. Likewise, the contact angles turnedout to be smaller in the order of: fluoroelastomer>poly(vinylchloride)>acrylic resin. It was understood therefrom that the greaterthe water repellency of the electroconductive resin layer, the morehighly durable will the fuel cell be.

EMBODIED EXAMPLE 3

An instance in which a resin having basic radicals is utilized will beexplained. In the present embodied example, a resin composition wasobtained utilizing flake graphite as an electroconductive particulatesubstance, and utilizing a poly(amide-imide) resin made by HitachiChemical Co., Ltd. as the resin and Cellosolve acetate as the solvent.The weight proportion of the flake graphite to the resin component inthis resin composition was 50:10. An electroconductive separator wasfabricated that, apart from utilizing this resin composition, was thesame as in Embodied Example 1, and the same evaluation was made. Theelectroconductive resin layer was made to be approximately 50 μm inthickness. As a result, the durability, though an initial voltage turnedout to be somewhat lower, improved greatly, compared to the cellsindicated in FIG. 2 in which the acrylic resin and poly(vinyl chloride)were utilized.

The initial voltage turning out to be lower is thought to be because theinitial electroconductivity of the electroconductive resin layer inwhich the poly(amide-imide) resin is utilized is lower than that of theelectroconductive resin layer in which the acrylic resin and poly(vinylchloride) are utilized.

EMBODIED EXAMPLE 4

When the single cell in Embodied Example 2 was operated more than 500hours, the output voltage dropped 10% or more compared to the start.Therein, an electroconductive separator was fabricated that, apart fromutilizing a vitreous carbon powder (approx. 20 μm mean particlediameter) instead of flake graphite, was the same as in Embodied Example2, and the same evaluation was made.

As a result, even with the elapse of 500 hours after the single cell waslaunched into operation, the voltage drop was less than 3%.Nevertheless, because the electroconductivity of vitreous carbon is lowcompared to flake graphite, the output voltage lowered slightly.

EMBODIED EXAMPLE 5

Respective electroconductive separators were fabricated that, apart fromutilizing, instead of flake graphite, acetylene black of 140 m²/g, 100m²/g, and 60 m²/g specific surface areas, were the same as in EmbodiedExample 2, and the same evaluation was made. As a result, it was evidentthat if the specific surface area is smaller than 100 m²/g, thesingle-cell's durability is efficaciously improved.

EMBODIED EXAMPLE 6

Instances in which a layer including metallic Zn, metallic Sn, metallicAl, a Cr-containing compound, a Mo-containing compound or a W-containingcompound was formed in between the metal substrate and theelectroconductive resin layer were investigated.

A Zn layer a few μm in thickness was formed by vapor deposition on thesurface of a sheet of carbon steel 500 μm in thickness. Anelectroconductive separator was fabricated that, apart from utilizingthis metal substrate, was the same as in Embodied Example 4, and thesame evaluation was made. Likewise, instances in which Sn and Al wereutilized in place of Zn were similarly investigated.

Furthermore, an instance was similarly evaluated in which a layer a fewμm in thickness of chromium oxide, molybdenum oxide and titanium oxidein place of the Zn layer was formed on the surface of the carbon-steelsheet by a sputtering method.

As a result, for whichever cell, the output drop following 500 runninghours elapsed post-start was in the 2% range. The initial voltages weresomewhat lower for the instances in which the layer of Al or oxides wasformed on the surface of the metal substrate. This is thought tooriginate in increased resistance due to the Al or oxide layer.

Further, instances in which resins, such as poly(vinyl chloride), thatdo not have water-repellent and basic properties were utilized in placeof the copolymer of tetrafluoroethylene and hexafluoropropylene weresimilarly investigated.

As a result, it was evident that forming a metal or oxide layer inbetween the metal substrate and the electroconductive resin layerimproved the electroconductive separator durability even wherein theelectroconductive resin layer did not have water-repellent and basicproperties.

EMBODIED EXAMPLE 7

Powders of 5 μm, 10 μm, 25 μm and 50 μm mean diameter were obtain bypulverizing vitreous carbon in a ball-mill. Electroconductive separatorswere fabricated that, apart from utilizing these powders aselectroconductive particulate substances, were the same as in EmbodiedExample 4, and the same evaluation was made. It was understood as aresult that the smaller the powder particle diameter and the greater thepowder content in the electroconductive resin layer, the higher theinitial voltage of the cell. Likewise, the larger the powder particlediameter and the smaller the powder content in the electroconductiveresin layer, the higher the durability of the cell.

INDUSTRIAL APPLICABILITY

The present invention enables in a fuel cell long-term control ofincrease in contact resistance of the gas-diffusion electrode andelectroconductive separator, and of output deterioration due tocorrosion in the electroconductive separator.

1. A polymer-electrolyte fuel cell, comprising: an electrolytemembrane-electrode assembly comprising a polymer-electrolyte membrane,and a pair of gas-diffusion electrodes sandwiching saidpolymer-electrolyte membrane; a first electroconductive separator havinga gas channel for supplying an oxidant gas to one of the gas-diffusionelectrodes of said pair; and a second electroconductive separator havinga gas channel for supplying a fuel gas to the other of the gas-diffusionelectrodes of said pair; wherein the polymer-electrolyte fuel cell ischaracterized in that at least one of said first electroconductiveseparator and said second electroconductive separator comprises a metalsubstrate and an electroconductive resin layer provided on said metalsubstrate and contacting said electrolyte membrane-electrode assembly,and said electroconductive resin layer comprises a resin having at leastone of water-repellant and basic radicals, and an electroconductiveparticulate substance which comprises a vitreous carbon powder having aspecific surface area of less than 100 m²/g and having a mean particlediameter of approximately 5 to 50 μm.
 2. The polymer-electrolyte fuelcell in accordance with claim 1, further comprising a layer including atleast one selected from the group consisting of: metallic Zn, metallicSn, metallic Al, Cr-containing compounds, Mo-containing compounds andW-containing compounds, between said metal substrate and saidelectroconductive resin layer.